1. Field of the Invention
This invention relates to momentum exchange actuator systems such as gyroscopes and the like, and more particularly to a reaction wheel for space craft applications.
2. Description of the Prior Art
Reaction wheels or similar inertial reference system components are employed to exert stabilization torques in control systems such as those required on spacecraft. Reaction wheels may further be used to cause the desired attitude changes of such vehicles. Since spacecraft may occupy space for considerable periods of time and may be unattended during this period, the multiple axis stabilization control systems must be active and alive for time periods in excess of 10 years. Further, the high precision pointing requirements of such vehicles require control devices with extremely low noise levels.
Where electrical power is continuously available, gas bearing rotors are attractive for long life advantages. Conical, spherical, or journal and thrust, vented or non-vented bearings with precision etched grooves in the bearing surfaces operate on a thin gas film which, of course, requires no lubrication or external pressure supplies. Theoretically, it is possible to operate indefinitely a hydrodynamic air or gas bearing without degradation, particularly where it is not permitted to "touch down", so that once relative rotation occurs between the wheel and its support, there is an absence of sliding contact. Recent developments in gas bearing technology have yielded significant advances in load capability which is essential for a saturated reaction wheel subject to rates normal to the spin axis.
Such wheels must be capable of being rotated in either direction or even stopped. This defeats the advantage of conventional hydrodynamic gas bearings, because when the bearing surfaces approach zero relative velocity, the hydrodynamic gas film support is lost and the surfaces rub and slide to a stop, causing a finite wear to the mechanism and eventual failure. Furthermore, because of the necessity for the wheel to be rotated bidirectionally, it is difficult to cause functional designs in terms of the etched grooves which will induce a superambient support pressure, regardless of the direction of rotation.
In contrast, conventional reaction wheel technology employs ball bearings on the rotation axis. Ball bearings have limited and somewhat unpredictable life spans, with the chief limitation residing in lubrication breakdown or failure of the lubricant retainer as by fracture or flaking.